In the U.S., telephone service is provided by multiple providers or carriers. Some calls are handled by one service provider, e.g., a local call completed between two wired telephone stations. Other calls, e.g., long-distance calls, can require multiple service providers to cooperate for their completion.
The initial carrier providing telephone service is a local service provider. Examples of such providers include a local exchange carrier (LEC), a cellular (wireless) carrier, cable and any other so-called "dial-tone provider" or "first contact service provider." The second carrier, in one example, is a long-distance--or so-called interexchange--telephone carrier. For example, presently, for a long-distance telephone call between wired telephone subscribers, the local service providers serving the calling and called telephones are LECs which are interconnected by a long-distance, or interexchange, carrier (IXC). In the U.S., AT&T is one of several IXCs.
In another example, the second carrier may be a LEC. For instance, for a local call from a mobile telephone to a wired telephone station, the local service provider is a cellular service provider and the second carrier could be a LEC that is connecting the network of the cellular service provider to a called party at the wired telephone station. Thus, it is possible for a telecommunications carrier, such as the LEC in the above two examples, to serve multiple roles, e.g., being the local service provider for some calls while being a second carrier for other calls.
Each telephone subscriber of a local service provider has associated with his or her telephone service one or more presubscribed default second carriers that were selected by or for the subscriber. One of these presubscribed default second carriers carries multiple-carrier calls originated from the subscriber's telephone unless the local service provider is instructed otherwise by way of the call-by-call entry of a telecommunications carrier access code indicating which telecommunications carrier should carry the particular call. For any call for which a telecommunications carrier code is not entered, the appropriate presubscribed default second carrier is typically identified based on the particular telephone link--e.g., wired telephone line, wireless (cellular), fiber optic or cable connection (or any other link provided to connect telephones to local service providers)--on which the local service provider received the call.
To facilitate the handling of long distance calls, each subscriber receiving local telephone service from a LEC has associated with his or her telephone line a "primary interexchange carrier," or PIC. The local subscriber is said to be "PIC'd", or presubscribed, to the associated IXC. The interexchange carrier to which a subscriber is presubscribed will be used by default to complete interexchange calls originated from his telephone lines unless the LEC is instructed otherwise. Such instruction is given when the subscriber enters, on a call-by-call basis, prior to dialing the called telephone number, a telecommunications carrier access code corresponding to the desired IXC. One such carrier access code is "10288" (1-0-A-T-T) corresponding to AT&T.
Specifically, when a long-distance call is initiated by the subscriber from his telephone line, and no access code is entered prior to the dialing of the called telephone number, the call is routed through the network of the originating subscriber's LEC, over the network of the presubscribed IXC and, ultimately through the network of the LEC which has as one of its subscribers the terminating subscriber. Alternatively, a subscriber may initiate a long-distance call by dialing, prior to supplying the called telephone number, a carrier access code that indicates which IXC he wishes to carry his call. When the LEC switch that serves the caller receives the carrier access code, it is recognized that the caller wishes his call to be carried over the IXC corresponding to the received carrier access code. Accordingly, the LEC routes the caller's call to a trunk-subgroup connecting the LEC to the IXC indicated by the carrier access code, regardless of the IXC to which the caller's telephone line is presubscribed.
It is noted that the supplying of a carrier access code typically indicates the selection by the caller of an IXC other than the one to which he is presubscribed (i.e., indicates that the caller "dialed around" his presubscribed carrier), but occasionally it may indicate the same IXC to which the caller is presubscribed. Information as to whether or not a caller who is routed to a particular IXC is presubscribed to that IXC is contained in a carrier selection information (CSI) parameter, included in the call signaling information.
Regardless of whether a caller uses a carrier access code to reach a particular IXC or is simply routed to the IXC to which he or she is presubscribed, a carrier identification code (CIC) designating the intended IXC for the call is included, according to, e.g., the ISDN user part (ISUP) CCSS7 protocol, in the call signaling information generated by the access provider. The CIC is used by the access provider to route calls to the correct IXC. Each CIC is unique to a particular IXC, although a given IXC may have more than one associated CIC. AT&T, for example, owns the 288, 686, and 732 carrier identification codes, among others.
It is known to those skilled in the art that CIC information, passed between access and long distance providers, can be used to route calls with different CICs on combined trunks, thereby realizing trunk savings. This is due to the fact that in the absence of per-call CIC information, network providers would have to maintain separate trunks between their networks to distinguish between individual CIC codes. This potential cost savings provides an incentive to network providers to negotiate with other network providers for the passing of per-call CIC information.